The oo vowel is a lip vowel and so the lips should be in a rounded position but not as closely rounded as the whistling OO lip position and the back of the tongue is raised. When I first started my training I had a problem with this sound - because my lips were too relaxed, when I tried to say “Look at the good book.” It sounded more like: “Lerk at the gerd berk.” Try to keep your lips nicely rounded for “oo” as in “book”